NPC Demographics

Adventurers - I rarely have this as a recognised profession IMCs, so no % of the population are recognised Adventurers.

Classed NPCs - in games which give NPCs classes, such as my Classic D&D Mystara campaign, I'll tend to use Gygax's standard that 1% of the population have classes and are capable of advancement. Other, static, NPCs are statted as appropriate, so eg in Classic trained soldiers are typically at least Fighter 1/1 hit dice, those being functionally identical concepts.

Spellcasters - In my Wilderlands game you typically see one Cleric type NPC per few hundred of the population, something under 0.5% but above 0.1%. Wizard type NPCs are much rarer, one in a thousand or less. A village of 400 might or might not have a priest with Clerical spellcasting, if not they probably have a ritualist priest with no combat casting.
My Golarion (Paizo's world) 5e campaign probably has a similar number of Clerical casters & somewhat higher numbers for arcanists.
Not sure about my Classic Mystara setting, but same sort of ballpark. Only the Alphatians field actual units of spellcasters, the (eg) Heldannic Knights field units of Fighters with Clerical support, at most roughly 1 Cleric per 10 Fighters in their case, & more like 1 per 100 for most armies.

Combatants - Wilderlands uses 25% combatant (2 hd+ in 5e, 1hd/Ftr-1 in Classic). When I ran Gygax's Yggsburgh setting in 1e AD&D I used his DMG numbers - 20% combatant, 10% in prime condition (hp 4+) and suitable as men-at-arms.

Ranking - My Yggsburgh game also used EGG's mercenary level distribution, with Ftr 2-3 lieutenants & Ftr 5-8 captains  as indicative of power level for mid-rank and powerful NPCs. Conversely my Mystara Classic game sees the Heldannic Knights field entire squadrons of elite Ftr-7 knights, and the King of Ostland's 20 'Brothers' were statted as Ftr-15 accompanying King Hord Darkeye Ftr-28 per the Northern Reaches GAZ, which says 1% of the population are Ftr-11+!

From http://www.dragonsfoot.org/forums/viewtopic.php?f=11&t=12309&p=1847970#p1847970